Did you know that species of fish change sex in adulthood?

Washington: Five hundred species of fish change sex in adulthood, often in response to environmental cues, researchers have found. "I've followed the bluehead wrasse for years because sex change is so quick and is triggered by a visual cue. How sex can reverse so spectacularly has been a mystery for decades. The genes haven't changed. So it must be the signals that turn them off and on," said Prof Jenny Graves, one of the researchers of the study.

Bluehead wrasses live in groups, on coral reefs of the Caribbean. A dominant male with a blue head protects a harem of yellow females. If the male is removed, the biggest female becomes male in just 10 days. She changes her behaviour in minutes and her colour in hours. Her ovary becomes a testis and by 10 days it is making sperm.

Using the latest genetic approaches and epigenetic analyses the researchers discovered that when and how specific genes are turned off so that sex change can occur. The study is important for understanding how genes get turned off and on during development in all animals including humans, and how the environment can influence this process.

"We found that sex change involves a complete genetic rewiring of the gonad. Genes needed to maintain the ovaries are first turned off, and then a new genetic pathway is steadily turned on to promote testis formation," said Dr Erica Todd, co-lead author of the study. Researchers said the amazing transformation also appears possible through changes in cellular "memory".

"Chemical markers on DNA control gene expression and to help cells remember their specific function in the body. Our study is important because it shows that sex change involves profound changes in these chemical marks," said Ortega-Recalde, one of the researchers of the study.

"With dragon lizards, the trigger for a sex change is temperature, which overrides genes on the male sex chromosomes and causes embryos to develop as females. Sex reversal in dragons and the wrasse involve some of the same genes. So I think we are looking at an ancient system for environmental control of gene activity," added Prof Graves.